MATHEMATICAL-MODEL OF TMA(-DEPENDENT SUBRETINAL HYDRATION IN CHICK RETINA() DIFFUSION AND PREDICTION OF LIGHT)

Purpose. To derive a mathematical model of TMA(+) diffusion across the retina that can be used to estimate the amplitude and kinetics of the light-evoked increase in subretinal hydration and its effect on the c oncentration of other ions. Methods. All experimental data were obtain ed in chick retina-pigment epithelium-choroid preparations as describe d in the accompanying paper.(5) Results. Diffusional properties of the retina were derived from the time course of [TMA(+)](0) in the subret inal space (SRS) after changes in the retinal perfusate. Then, the SRS volume changes underlying the light-induced [TMA(+)](0) response can be derived using a mathematical model of TMA(+) diffusion. Complete re tinal depth series of light-evoked [TMA(+)](0) responses could be simu lated by producing a corresponding expansion of the SRS. Volume change s inferred from the diffusion model were 2.2 to 3.8 times larger and m ore prolonged than could be derived directly from Delta[TMA(+)](0). Th e model predicted up to a 20% peak increase in subretinal-space hydrat ion during illumination. The effects of this Volume increase on subret inal K+ and Ca2+ were estimated. These predictions were supported by i nhibiting the volume increase with DIDS, which blocks retinal pigment epithelium basal membrane Cl- conductance. Conclusions. The primary so urce of light-evoked changes in extracellular TMA(+) concentration rec orded throughout the retina is an increase in hydration (volume) of th e subretinal space. The response spreads to the inner retina by diffus ion. Effects of TMA(+) diffusion lead to large underestimates of the u nderlying volume changes. The light-evoked volume change alters the co mposition of the subretinal space and light-induced responses of other ions.